A barrel start is a start used mainly in longer races in track and field. It is basically just a waterfall with an extra half-waterfall. The other half waterfall starts halfway across the track (in the 3rd lane of a 6 lane track, etc.) at the start of the one-turn stagger mark (for example, that of a 200 meter dash on a 400 meter track) and continues out to the outside lane. This is the barrel.
On this curved line, the lower ranked runners of the race start. The starting procedure is the same as that of a waterfall, except runners in the barrel must stay outside of the inside half of the track through the first turn.
A barrel start is usually used in longer races, where a waterfall would be used. The only difference is that when there are too many competitors to make a waterfall have a clean start, they throw a few racers in the barrel.